CO refer to the drive transistors. Any transistor driving an inductive load requires a flyweel (flyback) diode to protect it from the high reverse voltage. A fundamental property of an inductor is to resist a change in current, which is exactly what happens when the transistor turns off. It uses the energy in it's magnetic field to try and maintain the current, so the faster the switching action the faster the change in current and hence the higher the back emf seen by the transistor ( e = -Ldi/dt ). The diode allows the current to circulate and dissipate the energy and clamps the voltage to approximately 1V (0.7V is the norm for a fully forward biased diode).
So if the flywheel diode goes open circuit the transistor will work for a short time (how long depends on the load and the magnitude of the reverse voltage).
So in summary always ensure the flywheel diode is OK before replacing the switching transistor. As best practice I always change both at the same time.